1. Field of the Invention
This invention relates to a mobile television receiver or a vehicular television receiver.
2. Description of the Related Art
Some automotive vehicles are equipped with television receivers and related antennas. During the travel of such a vehicle, a received television signal tends to undergo fading. In some cases, fading disturbs images recovered from the received television signal. Usually, a space diversity reception system is effective to suppress such a disturbance in recovered images.
It is known to temporarily store a television signal in a memory frame by frame for the following purpose. In the case where a good receiving condition is replaced by a poor receiving condition, a display screen continues to indicate a frame represented by a television signal available in the good receiving condition rather than frames represented by a television signal available in the poor receiving condition.
Japanese published unexamined patent application 3-205965 discloses a television receiver with a diversity unit. The television receiver of Japanese application 3-205965 is designed to obtain a stable receiving image by providing a synchronizing signal generator for executing quickly phase leading-in of a synchronizing signal at the time of switching an antenna and at the time of switching a receiving channel. In the television receiver of Japanese application 3-205965, the diversity unit switches antennas for obtaining an optimal signal in receiving signals, and outputs an optimum receiving signal and antenna switching information. In addition, a television tuner outputs receiving channel switching information when a receiving channel is switched. Furthermore, a synchronizing signal generator separates a synchronizing signal from a video signal, and outputs a stable synchronizing signal in phase with a desired period part. The synchronizing signal generator increases an operating speed for correctly adjusting the phase of the synchronizing signal in the case where the antenna switching information or the receiving channel switching information is outputted. In such a way, it is possible to obtain an image whose horizontal and vertical synchronizations are stable.
Japanese published unexamined patent application 3-280674 discloses an on-vehicle television receiver designed to improve the quality of an indicated image. In the on-vehicle television receiver of Japanese application 3-280674, the generation of a ghost disturbance is detected. When the generation of a ghost disturbance is detected, an image available before the occurrence of the disturbance is indicated. Specifically, the on-vehicle television receiver of Japanese application 3-280674 includes a synchronizing signal separation circuit for separating a horizontal synchronizing signal from a received video signal, and a ghost fluctuation detection circuit for comparing a phase of the horizontal synchronizing signal with a phase of a reference signal to detect a phase error therebetween and to generate a ghost fluctuation signal representing a fluctuation in a time base of the horizontal synchronizing signal. A control circuit outputs a display control signal to a memory when the ghost fluctuation signal reaches a prescribed threshold level. In the case where the display control signal is outputted to the memory, a video signal which occurs before the outputting of the display control signal is read out from the memory before being fed to a display and being indicated thereon.
Japanese published unexamined patent application 5-328240 discloses a mobile body television receiver. Operation of the mobile body television receiver of Japanese application 5-328240 can be changed between a usual reception mode and a frame feed mode. During the usual reception mode of operation, a received TV signal is indicated on a display as it is. During the frame feed mode of operation, a signal corresponding to one field is repetitively indicated on a display. The mobile body television receiver of Japanese application 5-328240 includes a system control circuit which selects the frame feed mode of operation when a mean value of an electric field strength reaches a prescribed value or when a changing speed of the electric field strength reaches a prescribed speed. During the frame feed mode of operation, a 1-field-corresponding portion of the received TV signal is stored into a memory, and is then repetitively read out from the memory before being indicated on the display. The received TV signal stored in the memory is periodically updated at a frequency lower than the field frequency so that the received TV signal which is indicated on the display is thinned out during the frame feed mode of operation.